You seem to be worried that you will lose your British citizenship by virtue of renewing your Brazilian passport. Rest assured that this is exceedingly unlikely to happen. There are only a few cases in which British citizenship can be lost involuntarily:
- Fraud or misrepresentation during the naturalization/registration process (which doesn't apply to you, since you have British citizenship from birth), or
- The Home Secretary determines that "it would be conducive to the public good to deprive the person of his or her British nationality."
This last one seems awfully broad on its face, and the Home Office is a bit cagey about how it's used; but in practice it seems that it's largely applied to suspected terrorists and spies, ensuring that they cannot re-enter the UK. (See this article for a few more details.)
However, this is exceedingly unlikely to affect you. Moreover, there is no bar under either UK law or Brazilian law to holding two citizenships from birth. As far as I can tell, you will be able to hold both Brazilian and UK citizenships for as long as you choose to do so.